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According to The New York Times, here are the events, in order, that led to CIA director David Petraeus's downfall:

A family friend of Petraeus's started getting threatening emails from an anonymous sender.

This family friend reported the threatening emails to the FBI.

The FBI traced the origin of those emails to another person Petraeus knew, a woman who was his biographer.

The FBI gained access to that woman's regular email inbox, in order to look for more evidence.

In that inbox, the FBI found sexy emails from an unidentified sender using Gmail.

The FBI, for some reason, took the time to figure out that this unidentified sender using Gmail was (allegedly!) David Petraeus.

This discovery made the investigation relevant to national security, and the FBI made several disclosures of what it knew to public officials, leading to leaks and Petraeus's resignation.

Here's another timeline of the same events:

David Petraeus (allegedly!) creates a secret, unofficial Gmail account – or at least one that is, according to the Times, "not immediately identifiable" as his.

He (allegedly!) uses this secret Gmail account to send sexy emails to his mistress. (Presumably he assumes that he's taken reasonable precautions to avoid getting caught by his wife, her wife, or anyone else.)

He wakes up one summer morning, only to find out later that day that his mistress has her own anonymous email account, which she used to email someone in such a way that a third person felt harassed, that the FBI gained access to this account, discovered it belonged to his mistress, then looked at her normal email inbox, discovered sexy emails from an unidentifiable person, and finally, figured out that this secret – or, "not immediately identifiable" Gmail account – belonged to him.

In other words: due to allegations made by a friend of his against another friend – allegations that did not ultimately result in charges! – his secret email account and the emails sent from it, were not so secret anymore.

The lesson (other than that you should not have an affair and that you should not spend lots of time alone with someone you find attractive) is one all of us already know, but everyone seems to forget: Your emails are not as private as you think, and as soon as you send them, they exist forever, waiting to be discovered by someone you do not want reading them.

The same goes for your text messages, by the way. Just ask Tiger Woods.